1. Technical Field
The present invention relates to a method for fabricating semiconductor device, more particularly to a method for fabricating a semiconductor device with good optical yield.
2. Description of the Related Art
As the development of electronic products such as digital cameras and scanners progresses, the demand for image sensors increases accordingly. In general, image sensors in common usage nowadays are divided into two main categories: charge coupled device (CCD) sensors and CMOS image sensors (CIS). Primarily, CMOS image sensors have certain advantages of low operating voltage, low power consumption, and ability for random access. Furthermore, CMOS image sensors are currently capable of integration with the semiconductor fabrication process. Based on those benefits, the application of CMOS image sensors has increased significantly.
The CMOS image sensor separates incident light into a combination of light of different wavelengths. For example, the CMOS image sensor can consider incident light as a combination of red, blue, and green light. The light of different wavelengths is received by respective optically sensitive elements such as photodiodes and is subsequently transformed into digital signals of different intensities. Thus, it can be seen that a monochromatic color filter array (CFA) must be set above every optical sensor element for separating the incident light.
Typically, the color filter array is formed by photolithography process. In order to correctly transfer the patterns on the photo mask onto the substrate where the color filter array determined to be formed thereon, before the exposure process of each photolithography process is performed, it is necessary to perform an alignment process for aligning the photo mask to the substrate so that the improper pattern transfer will not happen. Usually, the alignment mark is formed on the substrate for forming scattering site or diffraction edge during the alignment process. Hence, while a light source is provided to illuminate the wafer, the diffraction patterns caused by the light beam passing by the alignment mark are reflectively projected onto the alignment sensor or onto the first-order diffraction interferometer alignment system.
Furthermore, since the backside image sensor including a color filter array and a lens array formed on backside has been widely used for increasing light sensitivity, how to perform an alignment process during the manufacturing process of the backside image sensor for improving the process yield is an important issue.